In 3rd generation partnership project, 3GPP, GSM/EDGE radio access network, GERAN, Release 7, a number of features for packet switched transmission were included in the specifications. One of these features is Dual Carrier Downlink.
Dual carrier downlink, see e.g. Sub clause 3.3.4 in 3GPP TS43.064 enables two carriers to be simultaneously transmitted to the mobile while still only one simultaneous carrier is supported for uplink, UL, transmission.
Although the 3GPP specifications does not specify the receiver architecture, the Dual Carrier Downlink feature in Rel-7 was implemented assuming an implementation with multiple narrowband receivers and consequently the number of carriers were restricted to two. Dual carrier transmission was only applied on the downlink, DL, and there was no impact to the radio performance requirements or radio related testing for the terminal.
However, two different architectures of mobile implementation were discussed prior to standardization, namely multiple narrowband receivers and wideband receiver.
In the feasibility study for 3GPP GERAN Rel. 7, both dual carrier and multicarrier transmission in both UL and DL were considered. It was recognized that for multicarrier solutions, where the number of carriers is high, a wideband receiver is better suitable while for lower number of carriers, e.g. 2, multiple narrowband receivers is a simple extension of the current single carrier architecture, resulting in no, or little, impact to the physical layer.
The decision of limiting the scope to dual carriers in the DL can be found in 3GPP TR 45.912, Sub clause 5.1, “Feasibility study for evolved GSM/EDGE Radio Access Network (GERAN)”:
“Given the current technical and implementation limitations, it is considered acceptable in an initial phase to restrict the number of carriers to two. The need for higher bit rates could make it desirable to support more than two carriers in future releases of the GERAN standards.”
The introduction of downlink dual carrier is an extension of the single carrier case and achievable gains, e.g. peak throughput, scale linearly with the number of carriers used, i.e. using two carriers doubles the peak throughput achievable. Further additional gains can be achieved by using even more carriers. For example using four carriers will double the peak throughput achieved with dual carrier implementations.
However, when implementing multicarrier transmission with a use of multiple narrowband receivers, each additional carrier would correspond to one additional radio frequency RF front end for each carrier supported, resulting in a bulky and expensive terminal. There is a need for a more cost efficient implementation of multicarrier data transmission.